A toner is a useful tool. Let's go back to basics, though.
First thing you need is an accurate 'map' of the house. Once you have that, go around the house and identify EVERYTHING. Identify the source of power for every receptacle, identify what every switch controls.
Besides making a drawing on paper, I also use colored masking tape to tag each cover plate. I start with blue, where I write the circuit #.
Once I have identified all I can, I go and check everything for proper operation. "Good" gets a bit of green masking tape. If there's a problem, a bit of red (get that mail order).
Finished with my 'survey,' I step back. Do I see any patterns?
Another tool in my arsenal is a 4-ft level. I set that against the wall at receptacle level, and I look for any bulges in the wall that might suggest a concealed box.
I look for signs of recent work, or repairs. I look for changes in texture. I try to guess how the building differs from its' original layout.
As a last resort, I'll power everything down and use a "data" toner to try to trace lines inside the walls.
Using these methods, I once was able to find several junction boxes that had been paved when a parking lot expanded. The boxes were about 6-ft away from the pavement edge.
